Water tank base



FIG. 1 is an isometric view of a water tank base showing our new design;

FIG. 2 is a front elevational view thereof

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a left side elevational view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof

The broken lines shown in the figure drawings depict portions of the water tank base which form no part of the claimed design. 

CLAIM The ornamental design for a water tank base, as shown and described. 